Steroidogenic factor-1 (SF-1), an orphan nuclear receptor, was studied with respect to the expression of steroidogenic enzymes in the hippocampus of rat and marmoset, since SF-1 is a regulator of steroid biosynthesis in the gonads. We used the steroidogenic acute regulatory protein (StAR) as a marker of the first step in the cascade of oestrogen synthesis and aromatase as a marker of the last. StAR transports cholesterol to the inner mitochondrial membrane where it is converted by the cytochrome P-450 enzyme complex. This is the rate-limiting step in steroid biosynthesis. Aromatase metabolizes testosterone to oestrogen. Using an anti-SF-1 antibody we show that SF-1 is highly expressed in neuronal cells of the pyramidal layer (CA1--CA3) and in the dentate gyrus of rat and marmoset hippocampi. Binding of the antibody was seen in more than 60% of all cells in the pyramidal layer and in the fascia dentata. In situ hybridization studies revealed the same expression pattern for StAR and aromatase. StAR and aromatase-positive cells were strictly correlated with SF-1 as shown by computer-assisted confocal microscopy in double labelling experiments (immunohistochemistry and in situ hybridization). This coexpression may imply SF-1 as a possible regulator of steroidogenesis in the hippocampus. However, a few interneurones express solely SF-1 and aromatase but are negative for StAR. Since the expression of StAR represents the first step in steroidogenesis its expression is suggestive for a de novo synthesis of steroids. A small population of interneurones must import precursors for oestrogen synthesis from other sources. Responsive cells, as evidenced by the presence of oestrogen receptor transcripts, were also found in the pyramidal layer and dentate gyrus. In conclusion, (1) SF-1 could play a regulatory role in steroidogenesis in the hippocampus of marmoset and rat and (2) with respect to the capacity of steroidogenesis two populations of hippocampal neurones coexist.